In conventional gas metal arc (GMAW) or metal-inert gas (MIG) welding, an electric current is supplied to a consumable electrode wire to create an arc between the tip of the electrode and a workpiece. Heat created by the arc causes the tip of the electrode to melt, thereby forming a droplet of molten metal. Due to the combination of electromagnetic and gravitational forces present, the droplet ultimately detaches and transfers along the arc to the workpiece. The electric arc and the transfer of droplets from the electrode create a weld pool. As the workpiece is traversed, successive weld pools create a weld bead, which is essential to the metal joining process. Additionally, to improve bead quality, an inert gas is generally provided around the arc. This gas shield serves to protect the metal droplet from the surrounding atmosphere as it transfers across the arc, thereby preventing in-flight oxidation and providing a bead of substantially the same composition as the electrode.
To achieve quality beads with an acceptable appearance, it is desirable to control the detachment of the droplets from the electrode. More specifically, the droplets must continuously reach an optimum size and detach with a known frequency. As can be appreciated, the size of each droplet and the detachment frequency is directly dependent on the level of current supplied to the electrode. It is known that droplet size at the time of detachment decreases as the current increases. This is primarily due to the increased electromagnetic detachment force associated with an increase in current. However, it is equally well known that high currents may also cause droplet superheating, which is associated with poor quality beads due to damaged material property. Such high currents also increase the presence of undesirable fumes and directly affect the mode of droplet detachment and transfer.
It is known that free, or natural, droplet detachment and transfer generally occurs in one of three distinct modes depending on the current level: short-circuit, globular, or spray transfer. In short-circuit transfer, the current remains too low to detach the droplet through electromagnetic force together with the weight of the droplet (or alternatively, the distance between the electrode wire and the workpiece, also known as the arc length, is too short). Thus, the droplet simply grows at a slow rate until it ultimately contacts the weld pool and detaches. This causes a "short-circuit" between the electrode and workpiece. Of course, short-circuiting will lower the arc voltage to zero. To maintain the arc voltage at the set level, the current must be increased. When the droplet is transferred to the weld pool (that is, when the surface tension breaks the liquid metal neck between the weld pool and the electrode tip is broken), the large current will cause the neck to explode and create undesired spatters.
If the current is increased sufficiently to grow the droplet, but remains too low to effect detachment, globular transfer results. In this transfer mode, detachment occurs when the weight of the droplet together with the electromagnetic force simply overcomes the surface tension at the liquid-solid interface (that is, the neck created between the molten metal droplet and the solid, unmelted electrode tip). As can be appreciated, globular transfer results in natural, but uncontrolled detachment, which creates undesirable spatter provides an overly broad weld pool and a nonuniform weld bead. Such uncontrolled detachment is not acceptable for most manufacturing operations, especially where the use of automated or semi-automated welding machines is contemplated.
Further increasing the current results in spray transfer, which can be subdivided into drop (projected) spray or streaming spray. Drop spray results when the current is sufficiently high to detach each droplet having a diameter close to that of the electrode. This current is known as the "transition" current. Drop spray provides the desirable characteristics of uniform droplet size, regular detachment, directional droplet transfer, and also creates little spatter. This produces a more uniform bead.
When the current is increased beyond the transition current, the transfer mode becomes streaming spray. This high current creates small droplets having an increased detachment frequency, both seemingly desirable characteristics. However, this transfer mode is known to create an undesirable finger-shaped penetration in the weld pool that is associated with poor mechanical properties. Also, the use of such a high current increases the presence of undesirable fumes and in many instances causes droplet superheating.
From consideration of the above description, it is clear that the preferred mode of free, natural transfer is drop spray. However, it should also be appreciated that the range of current in which this transfer mode is consistently achieved is extremely narrow. Further complicating matters is the potential for variations in the welding conditions, such as the electrode material, the composition of the shielding gas, and the electrode extension. The combination of a high, but narrow current range and the interdependence on welding conditions create two significant problems, namely: (1) droplet detachment is not always guaranteed for a given set of preselected welding parameters; and (2) the high current and concomitant heat input prevent use of GMAW in workpieces having particularly thin sections or comprised of relatively heat-sensitive materials.
In an effort to solve the aforementioned difficulties, others have proposed the use of a pulsed current, a technique that is well-known in the art. In pulsed GMAW, a base current maintains the arc, while a higher, peak current melts the tip of the electrode wire and detaches the droplet. The combination of base and peak currents over the pulse period results in a lower average current. Of course, this reduces the amount of heat input to the weld pool which, in turn, solves the second problem identified above.
However, the more difficult problem is the ability to consistently achieve drop spray at a lower current independent of the welding conditions presented. In pulsed current GMAW, it is desirable to achieve the detachment of one-drop-per-pulse (ODPP). Conventional pulsed current methods attempt to achieve ODPP by adjusting the duration of the peak current. However, to guarantee detachment and drop spray (that is, to avoid one-droplet multiple pulses (ODMP) or multiple-drops-per-pulse (MDPP)) using conventional methods, such as the method taught in the U.S. Pat. No. 3,683,149 to Mages, the peak current level must always, at a minimum, rise to the transition current level, regardless of changes in duration (see FIG. 7, main amplitude 51 of Mages). Of course, this is a high current level which requires relatively high energy input and could potentially increase fumes. Further, since high current causes the droplet to form and transfer very quickly, the instant of droplet detachment remains difficult to accurately control. This is particularly true in adaptive welding. Specifically, as welding conditions (e.g. the arc length and composition of the shielding gas) and welding parameters (e.g. the base current level and duration) change over the course of the welding operation, the optimum level and duration of the peak current must also change in order to achieve ODPP.
Previous attempts to solve this difficulty include detecting the exact instant of droplet detachment and instantaneously adjusting the current accordingly to provide ODPP. Such proposals include: (1) sensing the arc voltage (arc length) and current level to determine the detachment instant; and (2) detecting audio emissions created by the arc jump from the tip of the electrode to the root of the droplet. Once the detachment instant is determined, the current can be lowered to below the transition current to ensure that only a single droplet is detached, thereby preventing MDPP. However, as can be appreciated, these approaches still rely on natural droplet transfer and, therefore, must always utilize a current at least as high as the transition current. Also, despite these efforts, uncertainty as to the detachment instant and the accompanying droplet size remains.
Accordingly, a need is identified for an improved method of GMAW using pulsed current wherein the detachment of one-drop-per-pulse may be actively controlled. The method would use current levels below the transition current to effect droplet detachment, thereby avoiding the problems associated with the use of high current. Existing equipment would be used to implement the method.